Drilling systems are designed to drill into the earth to target hydrocarbon sources as efficiently as possible. Because of the significant financial investment required to reach and then extract hydrocarbons from the earth, drilling operators are under pressure to drill and reach the target as quickly as possible without compromising the safety of personal operating the drilling system. Typical drilling systems include a rig or derrick, a drill string supported by the rig, and a drill bit coupled to a downhole end of the drill string that is used to drill ther well into the earthen formation. Surface motors can apply torque to the drill string via a Kelly or top-drive thereby rotating the drill string and drill bit. Rotation of the drill string causes the drill bit to rotate thereby causing the drill bit to cut into the formation. Downhole or “mud motors” mounted in the drill string are used to rotate the drill bit independent from rotation of the drill string. Drilling fluid or “drilling mud” is pumped downhole through an internal passage of the drill string, through the downhole motor, out of the drill bit and is returned back to the surface through an annular passage defined between the drill string and well wall. Circulation of the drilling fluid removes cuttings from the well, cools the drill bit, and powers the downhole motors. Either or both the surface and the downhole motors can be used during drilling depending on the well plan. In any event, one measure of drilling efficiency is rate of penetration (ROP) (feet/hour) of the drill bit through the formation. The higher the ROP the less time is required to reach the target source. Because costs associated with drilling the well are pure expense to the drilling operator any decrease in the time needed to reach the target hydrocarbon source can potentially increase the return on investment required to extract hydrocarbons from that target source.
Directional drilling is a technique used to reach target hydrocarbons that are not vertically below the rig location. Typically the well begins vertically then deviates off of the vertical path at a kickoff point to turn toward the hydrocarbon source. Conventional techniques for causing slight deviations in the well include drill bit jetting and use of whipstocks. More prevalent directional drilling techniques, however, include steerable motors and rotary steerable systems. Steerable motors and rotary steerable systems are fundamentally different systems. Steerable motors use bent downhole motors to steer the rotating drill bit while the drill string slides, i.e. when the drill string does not rotate. As the drill bit rotates, the bent housing guides the drill bit in the direction of the bend. When the desired drilling direction is achieved, rotatory drilling resumes where the drill string and the drill bit rotate. Rotary steerable systems, in contrast, “push” or “point” the drill bit toward the predefined directions while the drill string and the drill bit rotate to define a turn in the well. Drillers will use steerable motors in lieu of other directional drilling techniques when higher build up rates (BURs) (degrees per 100 feet) are desirable. A higher BUR can effectuate a turn in a shorter distance and in a shorter period of time is therefore associated with a higher ROP through the turn. Lower build-up rates, indicative of more gradual turns and common to rotary steerable systems, may result in a lower ROP through the turn. But steerable motors are not without disadvantages. Using a steerable motor with a large bend during a rotary drilling mode can lead to failure of the downhole motor, the drill bit and other downhole tools. More severe bends increase the risk of failure. Lower bend angles decrease component failure risk but also decrease the build-up rate and can therefore decrease ROP.